Apparatus for the photo-initiated chemical cross-linking of material

ABSTRACT

The invention relates to an apparatus for the photo-initiated chemical cross-linking of material ( 28 ). To form one or more mouldings the material is enclosed in an optically transparent mould ( 26 ). The apparatus has at least one light source ( 12 ), for example a pulsed UV light source by which the material ( 28 ) can be acted upon by a light that triggers the cross-linking. The region to be cross-linked in the mould ( 26 ) is determined at least partially by beam-delimiting elements ( 20, 22 ) between the light source ( 12 ) and the mould ( 26 ). That can be achieved by arranging between the light source ( 12 ) and the mould ( 26 ) a mask ( 20 ) having transparent and non-transparent surface portions. The mask ( 20 ) is then projected onto the material ( 28 ) that is enclosed in the mould ( 26 ) by projection optics ( 24 ). The projection of the mask ( 20 ) onto the material is effected in a telecentric beam path.

The invention relates to an apparatus for the photo-initiated chemicalcross-linking of material that is enclosed in an optically transparentmould for the formation of one or more mouldings, which apparatus has atleast one light source by which the material can be acted upon by alight that triggers the cross-linking.

In the chemical cross-linking of material, the molecular structure ofthe material is altered by the joining of chains of molecules. Thematerial may undergo macroscopic changes during the process, for exampleit may change from one state of aggregation to a different state ofaggregation, for example from the liquid state to the solid state. Thecross-linking can be effected, for example, by irradiation withelectromagnetic waves (here generally referred to as light). In order totrigger the process, the photons must have a certain minimum energy,with the result that, typically, light in the ultraviolet (UV) waverange is used.

When such a liquid material is enclosed in an optically transparentmoulding tool, a chemical process that causes cross-linking in thematerial can be triggered by exposure to light. The material thenbecomes solid and retains the shape of the moulding tool. The materialmoulded in that manner is then removed from the moulding tool, solidmouldings of the desired shape being obtained.

When a mould is used in which a plurality of mouldings are to beproduced, it is expedient for those mouldings to be joined to oneanother in the mould. The liquid material that is to be cross-linked canthen be introduced at one site on the mould and is then able todistribute itself and fill up the entire mould. Problems occur, however,when the individual mouldings are to be separated from one another aftercross-linking. The parting sites often need to be processed thereafter,for example polished. The same problem occurs also in the case of onlyone moulding since the site at which the material to be cross-linked isintroduced often leaves traces on the moulding itself.

The problem underlying the invention is so to form the beam path in anapparatus of the type mentioned in the introduction that the light thattriggers the cross-linking assists in the formation of the outline ofthe moulding(s).

The problem underlying the invention is especially so to form the beampath in an apparatus of the type mentioned in the introduction that thesurfaces of the moulding(s) that are parallel to the optical axis of theapparatus are determined by the beam path of the light that triggers thecross-linking.

According to the invention, that problem is solved in that the region(s)to be cross-linked in the mould is/are determined at least partially bybeam-delimiting elements between the light source and the mould. As aresult, the region to be cross-linked can be actively controlled.

Such a beam-delimiting element may be, for example, a mask havingtransparent and non-transparent surface portions which is arrangedbetween the light source and the mould, the mask being projected ontothe material that is enclosed in the mould by projection optics.

A specific desired pattern is defined by the mask. That pattern thendetermines the radial shape of the moulding(s), relative to the opticalaxis of the apparatus. Even if the size of the mould is greater in thatradial direction, the material outside the pattern is not cross-linkedand can simply be rinsed off the moulding(s) after the moulding(s)has/have been removed.

Preferably, the light source should deliver as uniform as possible aflow of energy through the desired volume of the material to becross-linked and the total energy or intensity should be high enough forthe chemical process to be completed within as short a time as possible,enabling a good economically viable yield within a production process.

The light source may be optically approximately in point form andcondenser optics may be so arranged between the light source and themask that a largely homogeneous illumination of the mask is obtained.That measure is known and is not discussed in greater detail here.

The apparatus may be so constructed that the projection of the mask ontothe material is effected in a telecentric beam path. The telecentricbeam path may be either on the object-side side or on the image-side. Anobject-side telecentric beam path is defined as a beam path where theentrance pupil of the optical system lies at infinity, that is to saythe exit pupil coincides with the image focal point. An image-sidetelecentric beam path is defined as a beam path where the exit pupil ofthe optical system lies at infinity, that is to say the entrance pupilcoincides with the object focal point. When the telecentric beam path ison the image-side, the size of the image does not depend upon theposition of the image. In the present invention that is highlyadvantageous since the mouldings to be formed have a certain extent inthe direction of the optical axis of the system. Side surfaces of themouldings that extend parallel to the optical axis do not, as a result,suffer any curvature during cross-linking. Out-of-focus effects as aresult of varied distance and material thickness are minimised.

The light source may be a pulsed UV light source (flashlamp). The cwhigh performance halogen lamps (cw=continuous wave) that are typicallyused as light source, which take up a substantial amount of space, havean electrical output in the range of a few kW and are expensive, have ahigh rate of wear-and-tear, require complicated electrical control andhave a low UV yield. A pulsed light source has a very high UV componentcombined with a distinctly lower average electrical output. That stemsfrom the fact that the UV component is a function of the plasmatemperature of the arc and the total electrical output is restricted bythe material of the lamps (average thermal load limit). In pulsed lampsthe incandescent plasma is substantially hotter, that is to say thereare distinctly higher outputs within the individual light pulse. In suchlamps, the thermal shock resistance of the lamp is the only limitingfactor for the output in the individual pulse. The average electricaloutput is, on the other hand, low (for example a few watts). Such lampsare also cheaper. Typically, pulsed lamps achieve 200 to 300 times theUV component of cw-operated lamps.

A further advantage of pulsed lamps is that, owing to the lowerelectrical output, their construction volume is smaller. As a result,they can easily be integrated into an optical system and can be arrangedclosely together with the optical system in groups.

The pulsed-mode operation also enables exact dimensioning of the amountof irradiation in multiples of the individual pulses by simply countingthe pulses per projector.

A retroreflector may be arranged behind the mould, as viewed from thelight source The integration of a retroreflector into the beam pathafter the active volume has been irradiated causes the light to beprojected through the desired volume again. Since, typically, only asmall proportion is absorbed (that is to say, triggers the desiredchemical reaction) in a single passage of the beam, the degree ofefficiency of the overall arrangement is almost doubled. Moreover, theprojection back into the projector lamp massively increases the plasmatemperature of the discharge, which results in a desired intensificationof the UV component in the emission spectrum.

For the selection of a suitable energy range, either optical filters maybe integrated into the mask, or the collimator, projection optics ormoulding tool may be appropriately selectively transparent.

As a result of the present invention, inter alia the exposure to lightis homogeneous over relatively large surface areas and, moreover, can bealtered individually in fixed portions of the total surface area, whichis especially important in the case of multiple processing (that is tosay when a plurality of mouldings lie adjacent to one another in amould) and when small mouldings are being produced.

An embodiment of the invention is illustrated hereinafter in greaterdetail with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic representation of an embodiment of theinvention;

FIG. 2 is a diagrammatic representation of an arrangement of the opticalelements for producing an image-side telecentric beam path.

In FIG. 1 a projector is denoted by the reference numeral 10. Theprojector 10 contains a light source in the form of a flashlamp 12. Theflashlamp 12 is connected by way of leads 14 to an electrical control 16comprising a pulse counter. In the projector 10 there are arranged inthe beam path of the flashlamp 12, in succession, collimator optics 18,a mask 20 in the form of a transparency, a stop 22 and projection optics24. The stop 22 is arranged in the object focal plane of the projectionoptics 24.

The projector 10 is so arranged that it exposes to light a moulding tool26. Enclosed in the moulding tool is the material 28 that is to beexposed to light and cross-linked.

A retroreflector 30 is arranged behind the moulding tool 26, as viewedfrom the projector 10.

The optical axis of the apparatus is denoted by the reference numeral32. When a plurality of projectors are used, such an optical axis 32 isassociated with each projector. Reference to the optical axis 32, then,applies accordingly to each individual projector.

The stop 22 is so arranged in the projector 10 that it defines theentrance pupil. The stop 22 forms the aperture stop for the projectionoptics. An image-side telecentric beam path is produced as a result,which means that even relatively thick material 28, that is to saymaterial of large extent parallel to the optical axis 32, can be exposedto light well without distortion in the depth. Moreover, a greatermargin for manoeuvre in respect of the distance between the projector 10and the moulding tool 26 is possible without there being any negativeeffect on the outcome of the cross-linking.

As a result of the control 16 having a pulse counter, it is possible insimple manner to control the flow of energy through the material 28 thatis to be cross-linked.

For greater clarity, in FIG. 1 only one projector 10 is shown. Itshould, however, be mentioned expressly that the present invention isnot limited to the use of a single projector 10. Rather, it will beexpedient to use a plurality of projectors. If, for example, anarrangement of 4×5 mouldings is to be exposed to light with sharpdefinition in a transparent moulding tool, there may be used, forexample, 20 identically constructed projectors having pulsed lightsources and an average electrical output of approximately 40 W. Themouldings to be produced may be identical and, for example, may eachhave a diameter radially to the optical axis of 15 mm. The apparatus maybe so operated that the intensity reaches an average of 10 mW/cm² over aperiod of a few seconds. The masks 20 used may be metal masks which areprojected into the moulding tool. An estimation of the exposureintensity gives the following values:

degree process of efficiency output used flashlamp 10 W electricaloutput  70% 7 W collecting optics and projector  25% 1.75 W usable UVcomponent  3% 52 mW retroreflector 190% 100 mW total output  1% outputin active volumes per moulding 56 mW/cm²

The total electrical output required in that Example is a maximum of 200W from 20 individual lamps each of 10 W. In order to satisfy thoserequirements with known shadow-throwing technology and cw lamps, it isnecessary to use lamps having an electrical output of ≧2.5 kW.

The image-side telecentric beam path will now be described withreference to FIG. 2. Corresponding parts in FIG. 2 are provided with thesame reference numerals as in FIG. 1.

Arranged in succession on the optical axis 32 are the light source 12,the collimator optics 18, the mask 20, the aperture stop 22 and theprojection optics 24.The aperture stop 22 is arranged in the objectfocal plane of the projection optics 24. An image-side telecentric beampath is produced as a result. Two beams are denoted by the referencenumerals 36 and 38. The projection optics 24 project the mask 20 into animage plane 40. A sharp image of the mask 20 is obtained in that imageplane 40. Owing to the telecentric beam path there is obtained, both infront of and behind the image plane 40, an image of the mask 20 which,whilst not being sharp, does not, however, differ, or differs onlyinsubstantially, in size from the size of the image in the image plane40.

The collimator optics 18 project the light source 12 in the plane of theaperture stop 22. That is indicated by a small circle 34. A so-calledinterlinked or interwoven beam path is obtained in that manner, whicheffects good illumination of the mask 20.

What is claimed is:
 1. An apparatus for the photo-initiated chemicalcross-linking of material that is enclosed in an optically transparentmould for the formation of a moulding, comprising: a light source bywhich the material can be acted upon by light that triggers thecross-linking, said light source being optically approximately in pointform; a mask having transparent and non-transparent surface portions;condenser optics that are arranged between the light source and the maskin a manner so that a largely homogeneous illumination of the mask isobtained; and a projection optics for projecting the mask onto thematerial enclosed in the mould in an image-side telecentric beam path,wherein the region to be cross-linked in the mould is determined by theprojection of the mask by the projection optics onto the materialenclosed in the mould in the image-side telecentric beam path, therebydelimiting the radial shape of the moulding.
 2. An apparatus accordingto claim 1, wherein a retroreflector is arranged behind the mould, asviewed from the light source.
 3. An apparatus according to claim 1,wherein the light source is a pulsed UV light source.
 4. An apparatusaccording to claim 3, wherein a retroreflector is arranged behind themould, as viewed from the light source.